Contemporaneous Seawater Research Articles

Sr evolution in the Upper Permian and Lower Triassic carbonates, northeast Sichuan basin, China: Constraints from chemistry, isotope and fluid inclusions

Abstract Petrographic features, C, O and Sr isotopes, rare earth and trace elements were determined, and fluid inclusions were analyzed on various stages of interparticle cements and vug-fillings from the Upper Permian and Lower Triassic sour reservoirs in northeastern Sichuan basin. The aim was to assess the origin and evolution of palaeo-waters in the carbonates. The original water was contemporary seawater, from which marine cements precipitated with slightly high Sr contents (mean 1911 ppm), 87 Sr/ 86 Sr ratios from 0.7067 to 0.7082 and nonluminescent CL. The palaeo-seawater was diluted by meteoric water, as indicated by bright cathodoluminescence (CL) and Sr-depletion (0–516 ppm) in low-temperature calcite. When buried to temperatures of about 60–90 °C during Middle to Late Triassic, the palaeo-water was enriched in Sr released from the transformation of precursor aragonite and calcite to dolomite, resulting in precipitation of substantial pre-bitumen Sr-rich minerals (SrSO 4 and SrCO 3 ). For un-dolomitized limestone sections, aragonite neomorphism may have contributed Sr to the precipitation of small amounts of Sr-bearing minerals and calcite crystals with elevated homogenization temperatures (HTs, mainly from 90 to 130 °C) and wide Sr contents (from 34 to 3825 ppm), as recorded in stage III calcite. Since the Middle Jurassic, almost all of the early stage celestite and significant amounts of solid CaSO 4 have been consumed by reactions with hydrocarbons (i.e., TSR), resulting in water enriched in isotopically light CO 2 and HCO 3 - , Sr 2 + , Ba 2 + and Eu 2+ , as recorded in calcite with low δ 13 C values (down to −18.9‰), 87 Sr/ 86 Sr ratios from 0.7072 to 0.7076, high HTs (mainly 110–198 °C), positive Eu anomalies and high Sr and Ba contents. Subsequently, the water was uplifted and cooled down to about 115 °C, celestite and strontianite were precipitated with the occurrence of natural elemental S immiscible inclusions. TSR may have produced significant amounts of freshwater, which brought down Sr concentrations and salinities of the palaeo-waters to not more than about 6.0 wt.% NaCl equivalent.

Re–Os geochronology of black shales from the Neoproterozoic Doushantuo Formation, Yangtze platform, South China

In South China, the Doushantuo Formation represents 90% of the time interval of the Neoproterozoic Ediacaran Period (635–542Ma). However, subdivision and correlation of strata in this formation is limited by sporadic occurrences of fossils of uncertain age and a scarcity of radiometric time constraints. In order to obtain Re–Os ages and initial 187Os/188Os (Osi) data, black shale samples from the Doushantuo Formation (Yangtze Platform, South China) were analyzed using the CrO3-H2SO4 digestion technique. A Re–Os date of 595±22Ma (Osi=0.85±0.18, MSWD=29, n=7) was obtained for black shale samples from the basal part of the fossil-bearing Doushantuo Member 4 (“Miaohe Member”) at Jiulongwan section, Three Gorges area, South China. A subset of these samples define a more precise isochron age of 591.1±5.3Ma (Osi=0.874±0.038, MSWD=1.3, n=4). We interpret the high MSWD of 29 and larger uncertainties for the 595±22Ma date to reflect heterogeneity of the Osi of samples related to temporal change of 187Os/188Os ratio of seawater when the sedimentary rocks were deposited. Taking 591.1±5.3Ma as the age for the base of the Doushantuo Member 4, together with the age of a tuff bed on the top of Member 4 (551.1±0.7Ma, zircon U–Pb dating, Condon et al., 2005), a minimum time difference of 35m.y. between the base and top of Member 4 is obtained. This estimate suggests very low sedimentation rates during black shale deposition (∼0.4mm/ka). The time difference of 35m.y. also represents a minimum duration of the Shuram–Wonoka δ13C excursion in South China. Black shale samples from Doushantuo Member 2 at Baiguoyuan section yield an imprecise date of 592±68Ma (Osi=0.534±0.057, MSWD=77) due to limited spread in 187Re/188Os ratio of the samples and a relatively large sampled stratigraphic interval (∼6m). If deposition occurred in water masses with access to the open ocean, Osi of the hydrogenous fraction from these organic matter rich sedimentary rocks may record the Os isotopic composition of coeval seawater. Thus, combined with previously published Re–Os data, the highly radiogenic initial 187Os/188Os ratio of 0.874±0.038 derived from Re–Os isochron for the base of Doushantuo Member 4, and even more radiogenic values near the top of Member 4 (1.08–1.17, calculated at 551Ma) indicate a dramatic increase of the flux of 187Os into the ocean during the late Ediacaran. The enhanced 187Os flux can only result from enhanced continental weathering and oxidation rates during that period.

Late Dolomitization in Basinal Limestones of the Southern Apennines Fold and Thrust Belt (Italy)

The Triassic pelagic carbonates of the Lagonegro Units from the southern Apennines fold and thrust belt host discordant bodies of dolostone. These rocks show textural features typical of zebra or saddle dolomites. In this contribution, the results of field observations, petrography and geochemical analyzes performed on samples from three different outcrops are presented.Field data indicate that the peculiar zebra-type rock and brecciated fabrics were controlled by regular bedding of the micritic, pelagic limestones. The dolomitization resulted in the replacement of the host limestones and subsequent void-filling precipitation of dolomite in a dilatational stress field.Homogenization temperatures (Th) are in the range 80-120°C, with a clear mode of 95 ÷ 10°C. When corrected for maximum pressure, they indicate an upper limit for dolomite formation around 110-115°C. Melting temperatures of ice (Tmi ) point to salinities in the range 2-6 wt% NaCl eq, with a mean of 4.2%. δ 13 C values overlap those of the Upper Triassic seawater, whereas the δ 18 O values are significantly depleted compared to the coeval seawater. 87 Sr/86 Sr values are higher with respect to Upper Triassic seawater, and partially overlap Middle-Upper Miocene values.The performed analyzes indicate a dolomitization process driven by warm fluids (110-115°C) with a salinity close to that of seawater and O-isotope ratios comprised between seawater and formation waters. It is suggested that the dolomitization was accomplished by formation waters squeezed out or from surrounding lithologies or from the underlying melange units. Integration with available thermal data on the collisional history of the belt suggests that the fluid-flow took place after maximum burial, in the early stages of exhumation, in the assumption of a fluid in thermal equilibrium with the host rocks.According to literature data, the dolomitization event was quite widespread. It can be argued that it was part of a major fluid-flux associated with fold and thrust belt development, which possibly affected the Apulian Platform carbonates located in the foreland. The latter, now buried below the nappe stack, hosts some of the major oil fields of continental Europe.

SOURCE REGIONS AND THE SEDIMENTARY PALEOENVIRONMENT OF MARINE OIL SHALE FROM THE BILONG CO AREA, NORTHERN TIBET, CHINA: AN SR-ND ISOTOPIC STUDY

The Bilong Co oil shale zone is located in the South Qiangtang depression, northern Tibet, southwestern China. This zone, together with the Shengli River-Changshe Mountain oil shale zone in the North Qiangtang depression, northern Tibetan Plateau, potentially represents the largest marine oil shale resource in China. Altogether 18 samples of oil shale and micritic limestone were collected from the Bilong Co area to determine their Sr and Nd isotopic composition, as well as discuss sediment sources and paleoenvironmental changes. The Sm-Nd model ages for the Bilong Co oil shale samples support its derivation from the Nadi Kangra source whose model ages reflect the origin of volcanic rocks from the older continental crust by its melting. The Nd isotopic composition of the Bilong Co oil shale is similar to that of Late Triassic Nadi Kangri Formation volcanic-volcani- clastic rocks, which implies that this oil shale stems from said rocks. Generally, the vertical variations of eNd(0) values in the Bilong Co oil shale section are relatively homogeneous, suggesting a stable provenance for sediments. The 87 Sr/ 86 Sr ratio of the Bilong Co oil shale is much higher than that of contemporary seawater. Such a high 87 Sr/ 86 Sr ratio reflects the original com- position of the deposited sediment, both chemical and clastic. In the Bilong Co oil shale area, micritic limestone generally has a slightly lower 87 Sr/ 86 Sr ratio than oil shale. The Sr isotopic fluctuation in the Bilong Co oil shale is closely connected with sea level change. Sea level rise and consequent marine incursion may change the aquatic environment of the pre-existing lagoon, lowering the 87 Sr/ 86 Sr ratio of the precipitation fluid.

Strontium Isotope Ratios as a Tool for the Origin of Barite Mineralization of Marsis and Lefan Deposits/Northeast Zakho / Iraq

Veins, vienlets, pockets and cavity- filling deposits of barite are found to be associated with Upper Jurassic Formations (Marsis deposit) and Upper Cretaceous Bekhme Formation (Lefan deposit). The strontium isotope ratios of barite are presented here and the data are used to understand the source and origin of barite. The relation of barite mineralization with carbonate host rocks reveals a stratabound epigenetic origin. The obtained 87 Sr/ 86 Sr ratios range from 0.708226 to 0.708452; these values reveal the possibility of involvement of crustal Sr in the deposition of Marsis and Lefan barite. Isotopic data reveal that strontium in barite was derived from coeval seawater of Late Cretaceous and other processes like mixing of low temperature hydrothermal solution derived from the crust that was rich in radiogenic 87 Sr.

Phosphorite-hosted zinc and lead mineralization in the Sekarna deposit (Central Tunisia)

The Sekarna Zn–Pb deposit is located in Central Tunisia at the northeastern edge of the Cenozoic Rohia graben. Mineralization comprises two major ore types: (1) disseminated Zn–Pb sulfides that occur as lenses in sedimentary phosphorite layers and (2) cavity-filling zinc oxides (calamine-type ores) that crosscut Late Cretaceous and Early Eocene limestone. We studied Zn sulfide mineralization in the Saint Pierre ore body, which is hosted in a 5-m-thick sedimentary phosphorite unit of Early Eocene age. The sulfide mineralization occurs as replacements of carbonate cement in phosphorite. The ores comprise stratiform lenses rich in sphalerite with minor galena, Fe sulfides, and earlier diagenetic barite. Laser ablation–inductively coupled plasma mass spectrometry analyses of sphalerite and galena show a wide range of minor element contents with significant enrichment of cadmium in both sphalerite (6,000–20,000 ppm) and galena (12–189 ppm). The minor element enrichments likely reflect the influence of the immediate organic-rich host rocks. Fluid inclusions in sphalerite give homogenization temperatures of 80–130°C. The final ice melting temperatures range from −22°C to −11°C, which correspond to salinities of 15–24 wt.% NaCl eq. and suggest a basinal brine origin for the fluids. Sulfur isotope analyses show uniformly negative values for sphalerite (−11.2‰ to −9.3‰) and galena (−16‰ to −12.3‰). The δ34S of barite, which averages 25.1‰, is 4‰ higher than the value for Eocene seawater sulfate. The sulfur isotopic compositions are inferred to reflect sulfur derivation through bacterial reduction of contemporaneous seawater sulfate, possibly in restricted basins where organic matter was abundant. The Pb isotopes suggest an upper crustal lead source.

Dissolution methods for strontium isotope stratigraphy: Guidelines for the use of bulk carbonate and phosphorite rocks

Diverse carbonate dissolution methods have been applied to bulk carbonate rocks in order to target least altered components for strontium isotope stratigraphy (SIS). This is especially important for Precambrian and Cambrian studies for which no suitable skeletal material is available. One proven method for bulk limestones involves the removal of up to a third of the powdered sample using an acid pre-leach before partial dissolution of the rest of the sample using a weak acid or acid buffer solution. We applied a similar technique to dissolve a range of lithologies (limestone, dolostone and phosphorite) and compared the strontium isotopic composition of various leaches to contemporaneous seawater. Our results vindicate this approach and allow us to conclude that some dolomitic and phosphatic rock components may retain a near-primary seawater 87Sr/86Sr composition once contaminant strontium from secondary calcite and other phases has been removed. Commonly applied trace element (e.g. Mn/Sr, Mg/Ca) and isotopic (C, O) screening cut-offs were also examined in the light of these results, and proved unreliable particularly in the case of dolostones and partially dolomitized limestones. We recommend that rigid sample selection and sequential leaching procedures be applied to all SIS studies of bulk materials and propose a general protocol for strontium isotope studies on marine authigenic rocks which may also be applicable to rare earth element and other geochemical studies of marine authigenic minerals in the future.

Rare earth element and stable isotope (O, S) geochemistry of barite from the Bijgan deposit, Markazi Province, Iran

The Bijgan barite deposit, which is located northeast of Delijan in Markazi Province of Iran, occurs as a small lenticular body at the uppermost part of an Eocene volcano-sedimentary rock unit. The presence of fossiliferous and carbonaceous strata suggests that the host rocks were deposited in a quiet marine sedimentary environment. Barite, calcite, iron oxides and carbonaceous clay materials are found as massive patches as well as thin layers in the deposit. Barite is marked by very low concentrations of Sr (1–2%) and total amounts of rare earth elements (REEs) (6.25–17.39 ppm). Chondrite-normalized REE patterns of barite indicate a fractionation of light REEs (LREEs) from La to Sm, similar to those for barite of different origins from elsewhere. The LaCN/LuCN ratios and chondrite-normalized REE patterns reveal that barite in the Bijgan deposit is enriched in LREE relative to heavy rare earth elements (HREEs). The similarity between the Ce/La ratios in the barite samples and those found in deep-sea barite supports a marine origin for barite. Lanthanum and Gd exhibit positive anomalies, which are common features of marine chemical sediments. Cerium shows a negative anomaly in most samples that was inherited from the negative Ce anomaly of hydrothermal fluid that mixed with seawater at the time of barite precipitation. The δ18O values of barites show a narrow range of 9.1–11.4‰, which is close to or slightly lower than that of contemporaneous seawater at the end of the Eocene. This suggests a contribution of oxygen from seawater in the barite-forming solution. The δ34S values of barites (9.5–15.3‰) are lower than that of contemporaneous seawater, which suggests a contribution of magmatic sulfur to the ore-forming solution. The oxygen and sulfur isotope ratios indicate that submarine hydrothermal vent fluids are a good analog for solutions that precipitated barite, due to similarities in the isotopic composition of the sulfates. The available data including tectonic setting, host rock characteristics, REE geochemistry, and oxygen and sulfur isotopic compositions support a submarine hydrothermal origin for the Bijgan barite deposit. At the seafloor, barite deposition occurred where ascending Ba-bearing hydrothermal fluids encountered seawater. Sulfate was derived from the sulfate-bearing marine waters, and, to a lesser extent, by oxidized H2S, which was derived from magmatic hydrothermal fluids.

Carbon isotope chemostratigraphy and sedimentary facies evolution of the Ediacaran Doushantuo Formation in western Hubei, South China

The Doushantuo Formation of South China, consisting of intercalated siliciclastic and carbonate rocks which exhibit considerable variation in thickness and sedimentary facies, is one of the key sedimentary units in the world for understanding Ediacaran geobiological processes. However, stratigraphic correlation and sedimentary facies evolution of the Doushantuo Formation remain controversial. Sedimentary facies analysis of the well-known sections of the Yangtze Gorges area and new sections in northern Yichang, western Hubei indicate that the Doushantuo succession in the Yangtze Gorges area was deposited in an offshore intra-shelf basin within the Yangtze platform. In contrast, the Doushantuo Formation in the northern Yichang area which is characterized by one or a few phosphate-rich horizons represents deposition on the subtidal or intertidal shelf, and so is similar to the succession in the Weng’an area (central Guizhou Province). The carbon isotope profiles of the different sections exhibit significant variation, which is interpreted to result from facies differences. Sequence and high-resolution carbon isotope stratigraphy have clarified problematic correlations and the temporal relationship between fossil assemblages of the Doushantuo Formation on the Yangtze platform. The results confirm that a prominent negative δ13Ccarb excursion (BAINCE, nadir down to −5‰) in the middle portion of the second sequence of the Doushantuo Formation is a widely traceable event. BAINCE together with three other δ13Ccarb excursions, namely CANCE at the base, WANCE near the first sequence boundary near the middle, and DOUNCE at the top of the Doushantuo Formation, constitute a complete carbon isotope profile for the Doushantuo Formation that reflects contemporaneous seawater and so serves as a reference chemostratigraphic profile for global stratigraphic correlation and for evaluating the chemical evolution of seawater during the Ediacaran Period.

Molybdenum isotopic records across the Precambrian-Cambrian boundary

Molybdenum (Mo) isotopic data from early Cambrian formations in southern China demonstrate the importance of non-euxinic sediments as a promising archive for coeval seawater Mo records. For the first time, in analogue to noneuxinic, nonskeletal carbonates, we report that the pristine phosphorites also can preserve the ambient fluid, which make the pristine phosphorites a new potential fingerprinting tool to record the coeval paleoseawater. Pristine phosphorite and dolomite samples of two early Cambrian formations in southern China suggest that early Cambrian seawater may have had δ 97/95 Mo values of at least 1.4‰, similar to modern oceans. Low δ 97/95 Mo (as low as −0.5‰) values were obtained from reworked phosphorite, but evidence suggests that these are reflecting Mo related to Fe oxyhydroxide minerals rather than the coeval seawater. In contrast, late Precambrian dolomite just below the transition to the Cambrian would indicate a δ 97/95 Mo of coeval seawater slightly heavier than that of the Mesoproterozoic (0.9‰–1‰ versus 0.8‰), suggesting that oceanic circulation patterns may have been thoroughly reorganized by that time, and may have triggered biological diversification from the Ediacaran to the early Cambrian.

Geochemical characteristics of the barite deposits at cold seeps from the northern Gulf of Mexico continental slope

Although less common than the occurrence of authigenic carbonate, barite has been observed frequently at cold seeps on continental margins worldwide. It is understood that barite forms by the interaction of barium-rich and sulfate-free seeping fluids with dissolved sulfate of pore water near the seafloor, but questions remain about the geochemical processes and mode(s) of the barite formation. Here, we report geochemical characteristics of barite deposits at 11 cold seep locations from the northern Gulf of Mexico continental slope. Samples from these sites of fluid and gas expulsion provide environmental information on barite formation. Seafloor observations and samples acquired indicate that barites occur as chimneys, cones, crusts, irregular mound-like buildups up to 2-meters high, and as a material disseminated in host sediment. Most barite samples are white-to-gray and usually have a porous fabric and layered internal structure. Mineralogically, samples of barite may contain a significant amounts of carbonate minerals, such as calcite and dolomite, but aragonite is absent in all samples analyzed in this study. Negative δ 13C values (as low as − 46.4‰ V-PDB) of the associated carbonates strongly suggests that methane is the primary carbon source. The δ 34S and δ 18O values of the barites have large variations, ranging from 18‰ to 80.4‰ V-CDT, and 7.5‰ to 26.7‰ V-SMOW, respectively. On δ 34S versus δ 18O plots, many barite deposits show a linear trend that projects down toward the isotopic composition of seawater sulfate. The trend suggests that barite formed from seawater sulfate that has been isotopically modified to varying degrees by biological sulfate reduction. The δ 34S/δ 18O ratios vary between 2.4 and 4.1. The variations are interpreted to reflect local controls on the flux of barium-rich seep fluids, changes in the rate of bacterial sulfate reduction, and/or the openness of pore fluid system. The 87Sr/ 86Sr values of the barites indicate that within-site variation is small (< 0.00026) although there is a considerable range of Sr isotopic variations across multiple geographic sites (from 0.70782 to 0.71005). The observed variations probably reflect local controls on the source(s) and diagenetic evolution of seeping fluids. Strong deviation of the Sr isotope ratios of barites from coeval seawater ( 87Sr/ 86Sr = 0.70917) is interpreted as the modification of the strontium from less radiogenic sources like older marine sediments or more radiogenic terrigenous material such as basinal brine and/or meteoric water. The new results further offer a better understanding of the origin and geochemical history of barite deposits that occur in geological record on the basis of δ 34S and δ 18O compositions.

Age constraints on the Neoproterozoic Baikal Group from combined Sr isotopes and Pb–Pb dating of carbonates from the Baikal type section, southeastern Siberia

Depositional ages for the middle part of the Neoproterozoic sedimentary succession exposed along Lake Baikal in the southern margin of the Siberian Platform have been constrained by the application of Sr isotope chemostratigraphy and Pb–Pb dating of carbonate rocks. The thick siliciclastic–carbonate Baikal Group is the type section of the regional Siberian succession. Although many of the Baikal Group carbonate rocks have undergone pervasive dolomitization by mixed meteoric-marine fluids, oolitic limestones in the 800-m-thick carbonate member of the Uluntui Formation preserve primary textures and high Sr contents (up to 3200ppm), suggesting that aragonite might have been the original precursor. The 87Sr/86Sr in the least altered high-Sr limestones (Mg/Ca<0.08 and δ18O>−8‰ PDB) range within 0.70842–0.70876, values we interpret as representing the best proxy for coeval seawater. Stepwise dissolution using 0.5N HBr was applied to enrich samples in primary carbonate from the Uluntui Formation and nine data points from the five least altered, high-Sr limestones yielded a Pb–Pb isochron age of 560±30Ma with MSWD=0.6 and probability=0.8. This age matches that inferred from Sr isotope stratigraphy (580–550Ma) and further supports the inference that the Baikal paleobasin was connected to the contemporary oceans. Our Sr chemostratigraphy and Pb–Pb isochron age of the Uluntui limestones cast doubt on previously proposed correlations of the Baikal Group as late Mesoproterozoic to Cryogenian. Instead, the data indicate that the entire siliciclastic–carbonate succession of the Baikal Group was most likely deposited during the Ediacaran (or Vendian, of the Russian Stratigraphic Scale).

Molybdenum isotope constraints on the extent of late Paleoproterozoic ocean euxinia

The expansion of anoxic and sulfidic (euxinic) deep oceans may explain the termination of global large iron formation (IF) deposition at ~ 1.85 Ga, but the areal extent of late Paleoproterozoic ocean euxinia is poorly constrained. Here, we use the molybdenum (Mo) isotope composition (δ 98/95Mo) of black shales from the < 1.84 Ga Rove Formation (Animikie Group, Lake Superior region) to place constraints on the extent of late Paleoproterozoic ocean euxinia. Black shales from the lower Rove Formation have ratios of highly reactive Fe to total Fe (Fe HR/Fe T typically > 0.38) and ratios of pyrite Fe to highly reactive Fe (Fe PY/Fe HR typically < 0.8) that are consistent with a water column that was generally anoxic and Fe 2+-rich. Because the Fe PY/Fe HR ratios do not imply sustained water column euxinia, the conversion of molybdate (MoO 4 2–) to tetrathiomolybdate (MoS 4 2–) in bottom waters and sediments was probably not quantitative and thus the δ 98/95Mo (− 0.1 to + 0.8‰) from these shales provides only a minimum estimate for coeval seawater. By contrast, black shales from the middle Rove Formation have high Fe PY/Fe HR (> 0.8) and Fe HR/Fe T (> 0.38), which implies persistent water column euxinia. Concentrations of free sulfide may have been sufficient to result in near-quantitative formation of tetrathiomolybdate and its removal from bottom waters. The observed narrow range in δ 98/95Mo of 1.19 ± 0.28‰ (2SD) defined by most euxinic shales is thus inferred to represent the Mo isotope composition of late Paleoproterozoic global seawater. Mass-balance modeling suggests that organic-rich sediments deposited beneath euxinic bottom waters represented a substantial fraction of the oceanic Mo sink at 1.8 Ga. The modeling results suggest that euxinic marine environments were common along late Paleoproterozoic ocean margins. These environments could have served as efficient sinks for Fe 2+ (aq) upwelling from anoxic deeper waters. Hence, the Mo isotope data are consistent with Fe speciation and other geochemical evidence that suggest an expansion of euxinic environments led to the cessation of global large IF deposition after ~ 1.85 Ga.

Seawater Pb isotopes extracted from Cenozoic marine sediments

Nd and Pb isotopic compositions extracted from bulk deep sea sediments have been shown to be robust proxies for deep water circulation as well as weathering provenance and intensity over geologically young time scales. In this study we evaluated ten deep sea samples from Ocean Drilling Program (ODP) site 1090 ranging in age from mid Eocene to early-Miocene to test whether Pb isotopic compositions extracted from geologically older sediments record reliable seawater isotopic ratios and to evaluate the source of the extracted Pb. The sequential extraction protocol used in this study is similar to protocols reported for previous studies and produces acetic acid, hydroxylamine hydrochloride (HH) and residue fractions. Each extracted fraction was analyzed for Pb isotopes, rare earth elements (REEs), and a suite of major elements. Similar 206Pb/ 204Pb, 207Pb/ 204Pb, and 208Pb/ 204Pb ratios are recorded from the acetic acid and HH fractions for ~ 70–80% of the samples, suggesting that either the acetic acid dissolves Fe–Mn oxides or multiple phases are recording the same seawater isotopic value. Several indirect tests, such as Al mass balance, comparison of Sr isotopes in HH extracts to contemporaneous seawater Sr isotopes, and comparison of Nd isotopic compositions in HH extracts to published fossil fish teeth values, provide evidence that Pb isotopic compositions measured in our bulk HH extracts record bottom water values. The relationship between Pb, Mn and Ca concentrations in HH fractions indicates that Fe–Mn oxides and a Mn-bearing carbonate are the dominant phases contributing seawater Pb. Comparison of REE patterns derived from the HH fraction and total digestions of Fe–Mn nodule standards reveals that the trivalent REEs exhibit patterns consistent with the parent archive, but Ce can be fractionated during extraction. Ratios of REEs also produce unique fields for each fraction and can be used to test the purity of the seawater signal of the extraction protocol. Finally, an initial evaluation of Pb isotopic compositions in fossil fish indicates that this archive is not suitable for bottom water Pb isotope studies.

B, Sr, O and H Isotopic Compositions of Formation Waters from the Bachu Bulge in the Tarim Basin

Abstract In order to elucidate the origin and migration of basinal brines in the Bachu Bulge, Tarim Basin, we have carried out analyses on chemical composition, and boron, hydrogen and oxygen isotopes of formation waters together with the XRD of clay minerals from the Paleozoic strata. The waters show Ca, B, Li and Sr enrichment and SO4 depletion in the Carboniferous and Ordovician and K enrichment in part of the Ordovician relative to seawater. The relationship between δD and δ18O shows that all the data of the waters decline towards the Global Meteoric Water Line with the intersection of them close to the present-day local meteoric water, suggesting that modern meteoric water has mixed with evaporated seawater. The 87Sr/86Sr ratios range from 0.7090 to 0.7011, significantly higher than those of the contemporary seawater. The δ11B values range from +19.7 to +32.3‰, showing a decrease with the depth and B concentrations. The results suggest that isotopically distinct B and Sr were derived from external sources. However, since the percentages of illite are shown to increase with depth among clay minerals in the study area, i.e., illite is due to precipitation rather than leaching during deeper burial, it is unlikely for illite to have contributed a significant amount of B to the waters. Thus, B with low δ11B values is interpreted to have been added mainly from thermal degradation of kerogen or the basalts in the Cambrian and Lower Ordovician.

Molybdenum isotope evidence for mild environmental oxygenation before the Great Oxidation Event

We use the molybdenum isotope paleoredox proxy to look for evidence of small amounts of O 2 in the environment ∼50 Ma before the Great Oxidation Event (GOE) in a high resolution profile from the ∼2.5 Ga Mt. McRae Shale. The molybdenum isotope compositions (δ 98/95Mo) from samples throughout the sequence span a range from 0.99‰ to 1.86‰. All samples have heavier δ 98/95Mo compared to average upper continental crust. In addition, the upper (S1) and lower (S2) black shale units within the Mt. McRae Shale exhibit systematic differences in average isotopic compositions and distinct patterns of δ 98/95Mo variation. Heavier δ 98/95Mo values occur in the S1 unit, where δ 98/95Mo correlates with Mo enrichments. In the S2 unit, δ 98/95Mo is not as heavy and is relatively invariant. Based on sedimentary Fe proxies we infer that S1 sediments were deposited under euxinic conditions, so that Mo removal from bottom waters was likely quantitative. Thus, δ 98/95Mo in this interval likely records coeval seawater. The lighter δ 98/95Mo values in the S2 unit may indicate a less fractionated ocean Mo inventory relative to the S1 unit. However, sedimentary Fe proxies suggest that S2 sediments accumulated under a water column that was ferruginous rather than euxinic, raising the possibility of non-quantitative Mo scavenging and hence an expressed δ 98/95Mo fractionation relative to coeval seawater. Because any associated fractionations during this process would have favored the light isotope in sediments, the lighter δ 98/95Mo values in the S2 unit represent a lower limit on the value in contemporaneous seawater. After evaluating a range of hypotheses, we conclude that the isotopically heavy δ 98/95Mo values seen throughout the Mt. McRae Shale likely reflect the effects of oxidative weathering and adsorption of Mo to oxide mineral surfaces on land or in surface oceans. The extent of environmental oxygenation in either unit is difficult to assess due to uncertainties in the global Mo isotope budget. Because of the small ocean Mo inventory in the Late Archean, documented by low concentrations of Mo and low Mo/TOC, the extent of oxygenation required to account for the observed fractionations is much smaller than in modern oceans. However, when juxtaposed against the record of δ 98/95Mo through time, our findings provide further evidence of the onset of environmental oxygenation before the GOE.

EXTREME PGE CONCENTRATIONS IN LOWER CAMBRIAN ACID TUFF LAYER FROM THE KUNYANG PHOSPHATE DEPOSIT, YUNNAN PROVINCE, SOUTH CHINA--POSSIBLE PGE SOURCE FOR LOWER CAMBRIAN Mo-Ni-POLYELEMENT ORE BEDS

A study of the platinum group element (PGE) distribution in various lowermost Cambrian rock types of the Kunyang phosphate deposit in south China revealed unusually high PGE concentrations (434 ppb Pt, 142 ppb Pd, 57.97 ppb Rh, <1 ppb Ru, and 0.14 ppb Ir) in a whole-rock sample of acid tuff that is interbedded with phosphorite. Stratigraphically above this unit is an Mo-Ni–polyelement (including PGE and Au) phosphatic and sulfidic black shale. The tuff consists of clastic glass fragments in a matrix of clay minerals (dominantly mixed-layer illite-smectite) and cryptocrystalline quartz with minor dolomite, phosphatic fragments and nodules, and rare pyrite. The clay-quartz fraction revealed elevated Ru (11 ppb) and Ir (1.4 ppb) concentrations, but a depletion in Pt (6.8 ppb), Pd (25 ppb), and Rh (0.35 ppb) relative to the whole rock. LA-ICPMS analyses of pyrite yielded high Ni values (up to 1,243 ppm) and in some grains also elevated Re (up to 576 ppb) and Pd (up to 190 ppb) concentrations. No PGE were detected in the phosphatic nodules. It is very likely that the PGE are in part bound to submicroscopic inclusions in pyrite and/or intermetallic phases bound to the clay fraction. Petrographic observations and isotope data obtained on paragenetically late dolomite veinlets, which yielded C and O isotope ratios that correspond to those of contemporaneous seawater ( δ 13C ranges −2.32 to −2.44‰ V-PDB, δ 18O −10.07 to −9.94‰ V-PDB), do not indicate any hydrothermal overprint. Instead, pyrite formation is ascribed to diagenesis in the reduced environment of black shales, as supported by δ 34S ratios obtained for pyrite between 6.31 and 7.54 per mil CDT and elevated Ni concentrations in the pyrite. If other acid tuffs in the region have similar anomalous concentrations of PGE, they could have played an important role in the origin of the slightly younger Mo-Ni–polyelement (including PGE and Au) phosphatic and sulfidic black shales in the Yangtze platform.

Insights to magmatic–hydrothermal processes in the Manus back-arc basin as recorded by anhydrite

Microchemical analyses of rare earth element (REE) concentrations and Sr and S isotope ratios of anhydrite are used to identify sub-seafloor processes governing the formation of hydrothermal fluids in the convergent margin Manus Basin, Papua New Guinea. Samples comprise drill-core vein anhydrite and seafloor massive anhydrite from the PACMANUS (Roman Ruins, Snowcap and Fenway) and SuSu Knolls (North Su) active hydrothermal fields. Chondrite-normalized REE patterns in anhydrite show remarkable heterogeneity on the scale of individual grains, different from the near uniform REE N patterns measured in anhydrite from mid-ocean ridge deposits. The REE N patterns in anhydrite are correlated with REE distributions measured in hydrothermal fluids venting at the seafloor at these vent fields and are interpreted to record episodes of hydrothermal fluid formation affected by magmatic volatile degassing. 87Sr/ 86Sr ratios vary dramatically within individual grains between that of contemporary seawater and that of endmember hydrothermal fluid. Anhydrite was precipitated from a highly variable mixture of the two. The intra-grain heterogeneity implies that anhydrite preserves periods of contrasting hydrothermal versus seawater dominant near-seafloor fluid circulation. Most sulfate δ 34S values of anhydrite cluster around that of contemporary seawater, consistent with anhydrite precipitating from hydrothermal fluid mixed with locally entrained seawater. Sulfate δ 34S isotope ratios in some anhydrites are, however, lighter than that of seawater, which are interpreted as recording a source of sulfate derived from magmatic SO 2 degassed from underlying felsic magmas in the Manus Basin. The range of elemental and isotopic signatures observed in anhydrite records a range of sub-seafloor processes including high-temperature hydrothermal fluid circulation, varying extents of magmatic volatile degassing, seawater entrainment and fluid mixing. The chemical and isotopic heterogeneity recorded in anhydrite at the inter- and intra-grain scale captures the dynamics of hydrothermal fluid formation and sub-seafloor circulation that is highly variable both spatially and temporally on timescales over which hydrothermal deposits are formed. Microchemical analysis of hydrothermal minerals can provide information about the temporal history of submarine hydrothermal systems that are variable over time and cannot necessarily be inferred only from the study of vent fluids.

TSR origin of sulfur in Permian and Triassic reservoir bitumen, East Sichuan Basin, China

Solid bitumen, H 2S, pyrite and anhydrite samples from the Lower Triassic Feixianguan Formation and Upper Permian Changxing Formation, and source rock kerogen samples from the Cambrian, Ordovician, Silurian and Permian in the East Sichuan Basin were analyzed for their sulfur content and δ 34S values, to determine if the measurements were consistent with a thermochemical sulfate reduction (TSR) origin of the produced H 2S and solid bitumen. The results show that the solid bitumen samples have sulfur contents from 2.6 to 12.5 wt.% and δ 34S values from +13.4‰ to +22.1‰. The δ 34S values are significantly heavier than those of potential Upper Permian source rock kerogens that average +1.3‰ ( n = 3), suggesting that most of the sulfur in the solid bitumen is derived from isotopically heavy H 2S of TSR origin (from +12.0‰ to +13.7‰). Interestingly, two gases produced from gas to water transition zones during drill stem testing (DST) show significantly lighter δ 34S values (+3.3‰ and +4.1‰) than the H 2S from gas-producing intervals. The isotopically light H 2S may be genetically linked to fracture-filling isotopically heavy anhydrite with δ 34S values from +38.5‰ to +39.6‰, which are significantly heavier than those of coeval seawater sulfate (+11.8‰ to +21.7‰) as measured for bedded anhydrite. The link may have resulted from reduction of part of the dissolved sulfate by hydrocarbons under sulfate-limited conditions. Source rock kerogens show a secular change in δ 34S values relative to source rock age. A dramatic decrease and then increase in δ 34S kerogen was found toward the Permian/Triassic boundary for the first time in the East Sichuan Basin.

Platinum element group variations at the Permo–Triassic boundary in Kashmir and British Columbia and their significance

The end Permian marks the greatest extinction in the geological record, but there is no consensus on whether it was caused by terrestrial or extraterrestrial factors. Platinum group elements (PGE) can readily separate extraterrestrial from terrestrial sources and the few studies so far (from China and Europe) indicate terrestrial sources for the PGE. We report here detailed PGE analyses of two expanded sections from the late Permian northern and southern hemispheres that allow separate events to be distinguished and confirm a terrestrial source for the PGE. These sources are either contemporaneous seawater or older basaltic volcanics associated with the sections and the PGE were precipitated by a possible combination of development of anoxia in the oceans and post-depositional redistribution. The PGE content in sections at the Permo–Triassic boundary is the result of geochemical and lithological control on metal precipitation that is found in many other black shales throughout geological time.